1. Field of the Invention
The present invention relates to a synchronous electric machine having a stator with windings and a rotor with permanent magnets so that the rotor can be rotatable in the stator by mutual interaction between electromagnetic fields of the windings and magnetic fields of the permanent magnets.
2. Description of the Related Art
A synchronous electric machine of this kind is disclosed in Japanese patent laying-open publication No. Hei 07-255159. This electric machine acts as an electric motor and includes a rotor with permanent magnets to generate magnetic fields and a stator with windings to establish electromagnetic fields. The magnets are arranged at an uneven pitch along a circumference of the rotor so that the magnets are dislocated forward and backward by 30 degrees in an electric angle from the positions where they are placed at an even pitch. Specifically, the number of pole pairs is set to be an even number, one of pole pairs is set at 330 degrees in the electric angle and the other of pole pairs is set at 390 degrees in the electric angle. This causes that electromotive powers generated in U-phase windings are, for example, different forward and backward in phase between the two windings, so that their combined electric power changes in a step configuration, not in a rectangular configuration, in the rise and drop of the electromotive power. This brings the waveform of the electromotive power in each-phase winding to have a sinusoidal-like configuration, which in turn the terminal voltages between the U-, V-, and W- phases also have sinusoidal-like configurations. The sinusoidal-like configuration improves its control characteristics and running efficiency.
The above known conventional synchronous electric machine, however, encounters a problem that it can not decrease vibration or high-frequency electromagnetic noise of the stator caused by electromagnetic excitation forces acting on teeth of the stator. Because the conventional electric machine allows for only improvement of the control characteristics and running efficiency, which requires the dislocated angle of the magnets to be set ±30 degrees in the electric angle and 15 degrees in a mechanical angle.
It is, therefore, an object of the present invention to provide a synchronous electric machine which overcomes the foregoing drawbacks and can decrease electromagnetic excitation force acting on a stator and causing a high-frequency electromagnetic noise.